Olefin polymerization catalyst systems can be classified into Ziegler-Natta and metallocene catalyst systems, and these two highly active catalyst systems have been developed in compliance with their characteristics. Ziegler-Natta catalyst has been widely applied to existing commercial processes since it was developed in the 1950's. However, since the Ziegler-Natta catalyst is a multi-active site catalyst in which a plurality of active sites are mixed, it has a feature that the molecular weight distribution is broad. Also, since a compositional distribution of comonomers is not uniform, there is a problem that it has a limitation to secure the desired physical properties.
Meanwhile, the metallocence catalyst includes a main catalyst whose main component is a transition metal compound, and an organometallic compound cocatalyst whose main component is aluminum. Such a catalyst is a single-site catalyst which is a homogeneous complex catalyst, and offers a polymer having a narrow molecular weight distribution and an uniform composition distribution of comonomers, depending on the single site characteristics. The stereoregularity, copolymerizing properties, molecular weight, crystallinity and the like of the resulting polymer can be controlled by changing the ligand structure of the catalyst and the polymerization condition.
Among metallocene catalysts, an ansa-metallocene compound is an organic metal compound containing two ligands which are connected to each other by a bridge group, and the rotation of the ligand is prevented by the bridge group, and the activity and structure of the metal center are determined.
This ansa-metallocene compound is used as a catalyst in the preparation of an olefin-based homopolymer or copolymer. In particular, it is known that the ansa-metallocene compound containing cyclopentadienyl and fluorenyl can prepare a high molecular weight polyethylene, thereby controlling the microstructure of the polypropylene. Also, it is known that the ansa-metallocene compound containing indenyl ligand can prepare polyolefins having excellent activity and improved stereoregularity.
Meanwhile, CPP (casting polypropylene) film is a non-stretched film which is produced by T-Die process, and is widely used as a material for flexible packaging, such as food packaging. CPP film is generally composed of a skin layer, a core layer and a sealing layer. Here, the sealing layer is a portion where adhesion between the sealing layers is made by applying heat, and generally, polypropylene-based resin excellent in these properties is used.
However, the polypropylene-based resin prepared by Ziegler-Natta catalyst has high xylene soluble content and thus, a possibility of occurring defects in a process due to sticky properties is increased. Also, since the polypropylene-based resin prepared by metallocene catalysts has a relatively high melting point, there is a disadvantage that it is necessary to use high temperature for the adhesion between sealing layers.
Thus, the present inventors have conducted intensive studies about non-stretched polypropylene-based films having a low xylene soluble content and capable of allowing sufficient heat sealing at a low temperature, and found that a non-stretched polypropylene-based film comprising a terpolymer prepared by using a metallocene catalyst to be described below instead of a Ziegler-Natta catalyst and adjusting the comonomer, satisfies the above requirements, thereby completing the present invention.